1. Field of the Invention
The field of the invention is that of the treatment of industrial or urban effluents loaded with solid particles, especially but not exclusively sludge from purification facilities.
More generally, the invention applies to the treatment of effluents that contain a substantial proportion of organic matter and/or matter in suspension.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
The treatment in question consists in ridding the effluents to be treated of a substantial part of the undesirable compounds that they contain in order to discharge them into a natural receiving medium, a purification facility or a system. The effluent considered may be essentially water but it may also be any other industrial fluid to which the invention can be applied.
Typically, this treatment is implemented in a purification facility and seeks to process sludge coming from the process for the purification of the residual water that enters the purification facility. The treatment enables the conversion of the sludge into a suspension whose chemical oxygen demand (COD) is substantially reduced. The solid phase of said suspension which is highly mineralized may be discharged, and the aqueous phase of the suspension could be sent, as the case may be, to the head of the purification facility.
The purification methods used to treat urban or industrial effluents conventionally bring into play biological methods that are aimed at abating their biological oxygen demand (BOD) and that reproduce natural phenomena in accelerating them. However, certain effluents have pollutants that are hardly biodegradable and require the use of special methods and/or frequently require the use of chemical substrates.
One of the efficient types of treatment adapted to this application is that of wet air oxidation.
Wet air oxidation is a technique that has been much described in the prior art, especially in the U.S. Pat. No. 4,721,575 and 4,272,383 as well as for example the French patent FR 2 334 635. This technique is aimed at achieving an intensive oxidation of the organic matter contained in solutions having a high concentration of organic material that has low biodegradability or is not biodegradable. It has been implemented chiefly in the context of the treatment of industrial effluents and consists in placing an oxidizing gas in contact with said solution at a high temperature while at the same time keeping the solution in the liquid state. To this end, the conditions of the implementation of a method of this kind conventionally range from about 5 bars and 160 bars in pressure and from about 100.degree. C. and 350.degree. C. in temperature. The oxidizing gas used may be for example air, oxygen-enriched air or again molecular oxygen.
Other types of treatment exist, especially methods of purification by activated sludge and physical-chemical methods of precipitation. These treatments generally lead to the production of residual sludge, essentially consisting of non-soluble mineral compounds and non-decomposed organic materials.
In France, the quantity of sludge produced by the purification facilities is about one million tons of dry matter per year. About half of this sludge is reconverted for farm use while 35% is stored in waste dumps.
Subsequent to the setting up of new standards that purified waters must comply with, purification sludge is being produced in increasingly great quantities. At the same time, the regulations organizing the storage, agricultural reconversion or deterioration of this sludge are increasingly restrictive, given the fact that sludge of this kind could entail disadvantages to the environment and to health owing to its nature.
The improvement of treatment, with a view to resolving the problem of sludge, is a technological challenge to which the present invention provides a group of approaches. The treatment may in certain cases make it possible to achieve direct compliance with standards on rejection or reutilization.
Conventionally, it has essentially been sought to treat the sludge separately, independently of the process of treatment of the effluents. This treatment may consist in particular of prolonged aeration, anaerobic digestion, incineration, aerobic stabilization or again composting.
The purpose of this treatment especially is to reduce the dry matter content that it contains by oxidizing organic matter. However, the conventional sequences of sludge treatment, which generally include a step of mechanical dehydration have, in the majority of case, a negative influence on the efficient operation of the facility. The main reasons for this negative influence are related to the return of water from the sludge treatment to the head of the facility. This creates a excess load of COD as well as of nitrate to be eliminated by the facility. This excess load could amount to 15 to 30% of the initial load.
To try and resolve these problems, one approach consists in reinforcing the operational conditions of the treatment of the effluents used, in particular so as to accentuate the mineralization of the residual sludge. Thus, in the case of treatment by wet air oxidation, an attempt will be made to reinforce the mineralization by extending the time of the treatment or increasing the pressure, temperature or concentration in oxidizing gases.
However, this strategy tends to make the method very costly.